Electrodes, electrochemical cells, and methods of forming electrodes and electrochemical cells

ABSTRACT

Electrodes and methods of forming electrodes are described herein. The electrode can be an electrode of an electrochemical cell or battery. The electrode includes a current collector and a film in electrical communication with the current collector. The film may include a carbon phase that holds the film together. The electrode further includes an electrode attachment substance that adheres the film to the current collector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos.61/426,446, filed Dec. 22, 2010, and 61/488,313, filed May 20, 2011, theentirety of each of which is hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The present disclosure relates to electrochemical cells and electrodesused in electrochemical cells. In particular, the present disclosurerelates to electrodes and electrochemical cells that include silicon andcarbon composite materials for use in batteries.

2. Description of the Related Art

A lithium ion battery typically includes a separator and/or electrolytebetween an anode and a cathode. In one class of batteries, theseparator, cathode and anode materials are individually formed intosheets or films. Sheets of the cathode, separator and anode aresubsequently stacked or rolled with the separator separating the cathodeand anode (e.g., electrodes) to form the battery. For the cathode,separator and anode to be rolled, each sheet must be sufficientlydeformable or flexible to be rolled without failures, such as cracks,brakes, mechanical failures, etc. Typical electrodes includeelectro-chemically active material layers on electrically conductivemetals (e.g., aluminum and copper). For example, carbon can be depositedonto a current collector along with an inactive binder material. Carbonis often used because it has excellent electrochemical properties and isalso electrically conductive. Electrodes can be rolled or cut intopieces which are then layered into stacks. The stacks are of alternatingelectro-chemically active materials with the separator between them.

SUMMARY

In certain embodiments, an electrode is provided. The electrode caninclude a current collector and a film in electrical communication withthe current collector. The film may include a carbon phase that holdsthe film together. The electrode may also include an electrodeattachment substance that adheres the film to the current collector.

The film may be a monolithic self-supporting film. Furthermore, the filmmay include silicon particles distributed within the carbon phase. Thecarbon phase may include hard carbon. Furthermore, the film may includeporosity and at least some of the electrode attachment substance may bewithin the porosity of the film. For example, the porosity can be about5 to about 50 percent by volume of the film and/or about 1 to about 70percent by volume of the film.

The electrical attachment substance may include a polymer such aspolyamideimide, polyvinylidene fluoride, and polyacrylic acid.Furthermore, electrode attachment substance can be substantiallyelectrically nonconductive. The electrode attachment substance may allowfor expansion of the anode active material and current collector withoutsignificant failure of the electrode. For example, the electrode may beable to be bent to a radius of curvature of at least 7 mm withoutsignificant cracking.

In certain embodiments, a method of forming an electrode is provided.The method may include sandwiching an electrode attachment substancebetween a current collector and a solid film comprisingelectrochemically active material such that the electrode attachmentsubstance adheres the solid film to the current collector and the solidfilm is in electrical communication with the current collector. In someembodiments, the solid film at least partially absorbs the electrodeattachment substance into porosity of the film.

In certain embodiments, an electrochemical cell is provided. Theelectrochemical cell may include a porous separator sheet and a cellattachment substance sandwiched between the porous separator sheet andthe electrode described above. The cell attachment substance can includepolyvinylidene fluoride. The cell attachment substance may coat at leastone of or both of the porous separator sheet and the electrode. Forexample, the cell attachment substance that coats the porous separatorsheet can be a first cell attachment substance and the cell attachmentsubstance that coats the electrode can be a second cell attachmentsubstance that is chemically different than the first cell attachmentsubstance.

In certain embodiments, a method of forming an electrochemical cell isprovided. The method can include sandwiching a cell attachment substancebetween a porous separator sheet and the electrode described above. Themethod may further include coating at least one of or both of the porousseparator sheet and the electrode with the cell attachment substance.Moreover, the method may include heating the cell attachment substanceafter sandwiching the cell attachment substance between the porousseparator sheet and the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a method of forming a compositematerial that includes forming a mixture that includes a precursor,casting the mixture, drying the mixture, curing the mixture, andpyrolyzing the precursor;

FIG. 2 is a plot of the discharge capacity at an average rate of C/2.6;

FIG. 3 is a plot of the discharge capacity at an average rate of C/3;

FIG. 4 is a plot of the discharge capacity at an average rate of C/3.3;

FIG. 5 is a plot of the discharge capacity at an average rate of C/5;

FIG. 6 is a plot of the discharge capacity at an average rate of C/9;

FIG. 7 is a plot of the discharge capacity;

FIG. 8 is a plot of the discharge capacity at an average rate of C/9;

FIGS. 9A and 9B are plots of the reversible and irreversible capacity asa function of the various weight percentage of PI derived carbon from2611c and graphite particles for a fixed percentage of 20 wt. % Si;

FIG. 10 is a plot of the first cycle discharge capacity as a function ofweight percentage of carbon;

FIG. 11 is a plot of the reversible (discharge) and irreversiblecapacity as a function of pyrolysis temperature;

FIG. 12 is a photograph of a 4.3 cm×4.3 cm composite anode film withouta metal foil support layer;

FIG. 13 is a scanning electron microscope (SEM) micrograph of acomposite anode film before being cycled (the out-of-focus portion is abottom portion of the anode and the portion that is in focus is acleaved edge of the composite film);

FIG. 14 is another SEM micrograph of a composite anode film before beingcycled;

FIG. 15 is a SEM micrograph of a composite anode film after being cycled10 cycles;

FIG. 16 is another SEM micrograph of a composite anode film after beingcycled 10 cycles;

FIG. 17 is a SEM micrograph of a composite anode film after being cycled300 cycles;

FIG. 18 includes SEM micrographs of cross-sections of composite anodefilms;

FIG. 19 is a photograph of composite film showing wrinkles formed in thefilm as a result of cycling;

FIG. 20 is a photograph of a composite film without an electrodeattachment substance showing disintegration of the film as a result ofcycling;

FIG. 21 is a photograph of a composite film with an electrode attachmentsubstance of polyvinylidene fluoride (PVDF);

FIG. 22 is a photograph of a composite film with an electrode attachmentsubstance of polyamideimide (PAI);

FIG. 23 is a plot of gravimetric discharge capacity density as afunction of cycles for samples with different electrode attachmentsubstances and without an electrode attachment substance;

FIG. 24 is a plot of discharge capacity as percentage of 8th dischargecapacity as a function of cycles for samples with an electrodeattachment substance of PAI and without an electrode attachmentsubstance at an average charge rate of C and average discharge rate ofC;

FIG. 25 is a plot of discharge capacity as percentage of 8th dischargecapacity as a function of cycles for samples with an electrodeattachment substance of PAI and without an electrode attachmentsubstance at an average charge rate of C/5 and average discharge rate ofC/2;

FIG. 26 is a plot of discharge capacity as percentage of 2nd dischargecapacity as a function of cycles for samples with an electrodeattachment substance of PAI and samples with an electrode attachmentsubstance of PVDF at an average charge rate of C/5 and average dischargerate of C/5;

FIG. 27 is a plot of discharge capacity as a function of number ofcycles for cells with a cell attachment substance and cells without acell attachment substance;

FIG. 28 is a plot of discharge capacity as a function of number ofcycles for cells with a cell attachment substance comparing samples withdifferent separator materials;

FIG. 29 is a plot of discharge capacity as a function of number ofcycles for cells with a cell attachment substance comparing samples withdifferent electrolytes;

FIG. 30 is a photograph of an electrode showing wrinkling of an anodefilm;

FIG. 31A-C are photographs of anode films where pressure of (A) 100 lb,(B) 75 lb, and (C) 50 lb was applied to the cell; and

FIG. 32 is a photograph of an anode film which shows the absence ofwrinkles.

DETAILED DESCRIPTION

This application describes new electrodes (e.g., anodes and cathodes),electrochemical cells, and methods of creating electrodes andelectrochemical cells that may include a carbonized polymer. Forexample, a mixture that includes a precursor including silicon can beformed into a silicon composite material. This mixture includes bothcarbon and silicon and thus can be referred to as a silicon compositematerial as well as a carbon composite material. Examples of mixturesand carbon composite materials and carbon-silicon composite materialsthat can be used in certain electrodes, cells, and methods describedbelow are described in U.S. patent application Ser. No. 13/008,800,filed Jan. 18, 2011, and published on Jul. 21, 2011 as U.S. PatentApplication Publication No. 2011/0177393, entitled “Composite Materialsfor Electrochemical Storage,” the entirety of which is herebyincorporated by reference.

I. Composite Materials

Typical carbon anode electrodes include a current collector such as acopper sheet. Carbon is deposited onto the collector along with aninactive binder material. Carbon is often used because it has excellentelectrochemical properties and is also electrically conductive. If thecurrent collector layer (e.g., copper layer) was removed, the carbonwould be unable to mechanically support itself. Therefore, conventionalelectrodes require a support structure such as the collector to be ableto function as an electrode. The electrode (e.g., anode or cathode)compositions described in this application can produce electrodes thatare self-supported. The need for a metal foil current collector iseliminated or minimized because conductive carbonized polymer is usedfor current collection in the anode structure as well as for mechanicalsupport. A current collector may be preferred in some applications wherecurrent above a certain threshold is required. Methods of attachment ofthe composite film (e.g., piece) to a current collector are described insection II below. The carbonized polymer can form a substantiallycontinuous conductive carbon phase in the entire electrode as opposed toparticulate carbon suspended in a non-conductive binder in one class ofconventional lithium-ion battery electrodes. Advantages of a carboncomposite blend that utilizes a carbonized polymer can include, forexample, 1) higher capacity, 2) enhanced overcharge/dischargeprotection, 3) lower irreversible capacity due to the elimination (orminimization) of metal foil current collectors, and 4) potential costsavings due to simpler manufacturing.

Anode electrodes currently used in the rechargeable lithium-ion cellstypically have a specific capacity of approximately 200 milliamp hoursper gram (including the metal foil current collector, conductiveadditives, and binder material). Graphite, the active material used inmost lithium ion battery anodes, has a theoretical energy density of 372milliamp hours per gram (mAh/g). In comparison, silicon has a hightheoretical capacity of 4200 mAh/g. Silicon, however, swells in excessof 300% upon lithiation. Because of this expansion, anodes includingsilicon may expand/contract and lose electrical contact to the rest ofthe anode. Therefore, a silicon anode should be designed to be able toexpand while maintaining good electrical contact with the rest of theelectrode.

This application also describes a new method of creating monolithic,self-supported anodes using a carbonized polymer. Because the polymer isconverted into a electrically conductive and electrochemically activematrix, the resulting electrode is conductive enough that a metal foilor mesh current collector can be omitted or minimized. The convertedpolymer also may act as an expansion buffer for silicon particles duringcycling so that a high cycle life can be achieved. In certainembodiments, the resulting electrode is an electrode that is comprisedsubstantially of active material. In further embodiments, the resultingelectrode is substantially active material. The electrodes can have ahigh energy density of between about 500 mAh/g to about 1200 mAh/g thatcan be due to, for example, 1) the use of silicon, 2) elimination orsubstantial reduction of metal current collectors, and 3) beingcomprised entirely (or almost entirely) of active material.

The composite materials described herein can be used as an anode in mostconventional lithium ion batteries; they may also be used as the cathodein some electrochemical couples with additional additives. The compositematerials can also be used in either secondary batteries (e.g.,rechargeable) or primary batteries (e.g., non-rechargeable). In certainembodiments, the composite materials are self-supported structures. Infurther embodiments, the composite materials are self-supportedmonolithic structures. For example, a collector may not be included inthe electrode comprised of the composite material. In certainembodiments, the composite material can be used to form carbonstructures discussed in U.S. Patent Application Publication No.2011/0020701 entitled “Carbon Electrode Structures for Batteries,” theentirety of which is hereby incorporated by reference. Furthermore, thecomposite materials described herein can be, for example, siliconcomposite materials, carbon composite materials, and/or silicon-carboncomposite materials.

FIG. 1 illustrates one embodiment of a method of forming a compositematerial 100. For example, the method of forming a composite materialcan include forming a mixture including a precursor, block 101. Themethod can further include pyrolyzing the precursor to convert theprecursor to a carbon phase. The precursor mixture may include carbonadditives such as graphite active material, chopped or milled carbonfiber, carbon nanofibers, carbon nanotubes, and/or other carbons. Afterthe precursor is pyrolyzed, the resulting carbon material can be aself-supporting monolithic structure. In certain embodiments, one ormore materials are added to the mixture to form a composite material.For example, silicon particles can be added to the mixture. Thecarbonized precursor results in an electrochemically active structurethat holds the composite material together. For example, the carbonizedprecursor can be a substantially continuous phase. The silicon particlesmay be distributed throughout the composite material. Advantageously,the carbonized precursor will be a structural material as well as anelectro-chemically active and electrically conductive material. Incertain embodiments, material particles added to the mixture arehomogenously distributed throughout the composite material to form ahomogeneous composite.

The mixture can include a variety of different components. The mixturecan include one or more precursors. In certain embodiments, theprecursor is a hydrocarbon compound. For example, the precursor caninclude polyamic acid, polyimide, etc. Other precursors include phenolicresins, epoxy resins, and other polymers. The mixture can furtherinclude a solvent. For example, the solvent can be N-methyl-pyrollidone(NMP). Other possible solvents include acetone, diethyl ether, gammabutyrolactone, isopropanol, dimethyl carbonate, ethyl carbonate,dimethoxyethane, etc. Examples of precursor and solvent solutionsinclude PI-2611 (HD Microsystems), PI-5878G (HD Microsystems) and VTECPI-1388 (RBI, Inc.). PI-2611 is comprised of >60% n-methyl-2-pyrollidoneand 10-30% s-biphenyldianhydride/p-phenylenediamine. PI-5878G iscomprised of >60% n-methylpyrrolidone, 10-30% polyamic acid ofpyromellitic dianhydride/oxydianiline, 10-30% aromatic hydrocarbon(petroleum distillate) including 5-10% 1,2,4-trimethylbenzene. Incertain embodiments, the amount of precursor (e.g., solid polymer) inthe solvent is about 10 wt. % to about 30 wt. %. Additional materialscan also be included in the mixture. For example, as previouslydiscussed, silicon particles or carbon particles including graphiteactive material, chopped or milled carbon fiber, carbon nanofibers,carbon nanotubes, and other conductive carbons can be added to themixture. In addition, the mixture can be mixed to homogenize themixture.

In certain embodiments, the mixture is cast on a substrate, block 102 inFIG. 1. In some embodiments, casting includes using a gap extrusion or ablade casting technique. The blade casting technique can includeapplying a coating to the substrate by using a flat surface (e.g.,blade) which is controlled to be a certain distance above the substrate.A liquid or slurry can be applied to the substrate, and the blade can bepassed over the liquid to spread the liquid over the substrate. Thethickness of the coating can be controlled by the gap between the bladeand the substrate since the liquid passes through the gap. As the liquidpasses through the gap, excess liquid can also be scraped off. Forexample, the mixture can be cast on a polymer sheet, a polymer roll, orfoils or rolls made of glass or metal. The mixture can then be dried toremove the solvent, block 103. For example, a polyamic acid and NMPsolution can be dried at about 110° C. for about 2 hours to remove theNMP solution. The dried mixture can then be removed from the substrate.For example, an aluminum substrate can be etched away with HCl.Alternatively, the dried mixture can be removed from the substrate bypeeling or otherwise mechanically removing the dried mixture from thesubstrate. In certain embodiments, the dried mixture is a precursor filmor sheet. In some embodiments, the dried mixture is cured, block 104. Ahot press can be used to cure and to keep the dried mixture flat. Forexample, the dried mixture from a polyamic acid and NMP solution can behot pressed at about 200° C. for about 8 to 16 hours. Alternatively, theentire process including casting and drying can be done as aroll-to-roll process using standard film-handling equipment. The driedmixture can be rinsed to remove any solvents or etchants that mayremain. For example, de-ionized (DI) water can be used to rinse thedried mixture. In certain embodiments, tape casting techniques can beused for the casting. In other embodiments, there is no substrate forcasting and the anode film does not need to be removed from anysubstrate. The dried mixture may be cut or mechanically sectioned intosmaller pieces.

The mixture further goes through pyrolysis to convert the precursor tocarbon, block 105. In certain embodiments, the mixture is pyrolysed in areducing atmosphere. For example, an inert atmosphere, a vacuum and/orflowing argon, nitrogen, or helium gas can be used. In some embodiments,the mixture is heated to about 900° C. to about 1350° C. For example,polyimide formed from polyamic acid can be carbonized at about 1175° C.for about one hour. In certain embodiments, the heat up rate and/or cooldown rate of the mixture is about 10° C./min. A holder may be used tokeep the mixture in a particular geometry. The holder can be graphite,metal, etc. In certain embodiments, the mixture is held flat. After themixture is pyrolysed, tabs can be attached to the pyrolysed material toform electrical contacts. For example, nickel, copper or alloys thereofcan be used for the tabs.

In certain embodiments, one or more of the methods described herein is acontinuous process. For example, casting, drying, curing and pyrolysiscan be performed in a continuous process; e.g., the mixture can becoated onto a glass or metal cylinder. The mixture can be dried whilerotating on the cylinder creating a film. The film can be transferred asa roll or peeled and fed into another machine for further processing.Extrusion and other film manufacturing techniques known in industrycould also be utilized prior to the pyrolysis step.

Pyrolysis of the precursor results in a carbon material (e.g., at leastone carbon phase). In certain embodiments, the carbon material is a hardcarbon. In some embodiments, the precursor is any material that can bepyrolysed to form a hard carbon. When the mixture includes one or moreadditional materials or phases in addition to the carbonized precursor,a composite material can be created. In particular, the mixture caninclude silicon particles creating a silicon-carbon (e.g., at least onefirst phase comprising silicon and at least one second phase comprisingcarbon) or silicon-carbon-carbon (e.g., at least one first phasecomprising silicon, at least one second phase comprising carbon, and atleast one third phase comprising carbon) composite material. Siliconparticles can increase the specific lithium insertion capacity of thecomposite material. When silicon absorbs lithium ions, it experiences alarge volume increase on the order of 300+ volume percent which cancause electrode structural integrity issues. In addition to volumetricexpansion related problems, silicon is not inherently electricallyconductive, but becomes conductive when it is alloyed with lithium(e.g., lithiation). When silicon de-lithiates, the surface of thesilicon losses electrical conductivity. Furthermore, when siliconde-lithiates, the volume decreases which results in the possibility ofthe silicon particle losing contact with the matrix. The dramatic changein volume also results in mechanical failure of the silicon particlestructure, in turn, causing it to pulverize. Pulverization and loss ofelectrical contact have made it a challenge to use silicon as an activematerial in lithium-ion batteries. A reduction in the initial size ofthe silicon particles can prevent further pulverization of the siliconpowder as well as minimizing the loss of surface electricalconductivity. Furthermore, adding material to the composite that canelastically deform with the change in volume of the silicon particlescan ensure that electrical contact to the surface of the silicon is notlost. For example, the composite material can include carbons such asgraphite which contributes to the ability of the composite to absorbexpansion and which is also capable of intercalating lithium ions addingto the storage capacity of the electrode (e.g., chemically active).Therefore, the composite material may include one or more types ofcarbon phases.

Embodiments of a largest dimension of the silicon particles includesless than about 40 μm, less than about 1 μm, between about 10 nm and 40μm, between about 10 nm and 1 μm, less than about 500 nm, less thanabout 100 nm, and about 100 nm. All, substantially all, or at least someof the silicon particles may comprise the largest dimension describedabove. For example, an average or median largest dimension of thesilicon particles include less than about 40 μm, less than about 1 μm,between about 10 nm and 40 μm, between about 10 nm and 1 μm, less thanabout 500 nm, less than about 100 nm, and about 100 nm. The amount ofsilicon in the composite material can be greater than zero percent byweight of the mixture and composite material. In certain embodiments,the amount of silicon in the mixture is between greater than 0% and lessthan about 90% by weight or between about 30% and about 80% by weight ofthe mixture. Embodiments of the amount of silicon in the compositematerial include greater than 0% and less than about 35% by weight,greater than 0% and less than about 25% by weight, between about 10 andabout 35% by weight, and about 20% by weight. In further certainembodiments, the amount of silicon in the mixture is at least about 30%by weight. Additional embodiments of the amount of silicon in thecomposite material include more than about 50% by weight, between about30% and about 80% by weight, between about 50% and about 70% by weight,and between about 60% and about 80% by weight. Furthermore, the siliconparticles may or may not be pure silicon. For example, the siliconparticles may be substantially silicon or may be a silicon alloy. In oneembodiment, the silicon alloy includes silicon as the primaryconstituent along with one or more other elements.

The amount of carbon obtained from the precursor can be about 50 weightpercent from polyamic acid. In certain embodiments, the amount of carbonfrom the precursor in the composite material is about 10 to 25% byweight. The carbon from the precursor can be hard carbon. Hard carbon isa carbon that does not convert into graphite even with heating in excessof 2800 degrees Celsius. Precursors that melt or flow during pyrolysisconvert into soft carbons and/or graphite with sufficient temperatureand/or pressure. Hard carbon may be selected since soft carbonprecursors may flow and soft carbons and graphite are mechanicallyweaker than hard carbons. Other possible hard carbon precursors includephenolic resins, epoxy resins, and other polymers that have a very highmelting point or are crosslinked. Embodiments of the amount of hardcarbon in the composite material includes about 10% to about 25% byweight, about 20% by weight, and more than about 50% by weight. Incertain embodiments, the hard carbon phase is substantially amorphous.In other embodiments, the hard carbon phase is substantiallycrystalline. In further embodiments, the hard carbon phase includesamorphous and crystalline carbon. The hard carbon phase can be a matrixphase in the composite material. The hard carbon can also be embedded inthe pores of the additives including silicon. The hard carbon may reactwith some of the additives to create some materials at interfaces. Forexample, there may be a silicon carbide layer between silicon particlesand the hard carbon.

In certain embodiments, graphite particles are added to the mixture.Advantageously, graphite is an electrochemically active material in thebattery as well as an elastic deformable material that can respond tovolume change of the silicon particles. Graphite is the preferred activeanode material for certain classes of lithium-ion batteries currently onthe market because it has a low irreversible capacity. Additionally,graphite is softer than hard carbon and can better absorb the volumeexpansion of silicon additives. In certain embodiments, the largestdimension of the graphite particles is between about 0.5 microns andabout 20 microns. All, substantially all, or at least some of thegraphite particles may comprise the largest dimension described herein.In further embodiments, the average or median largest dimension of thegraphite particles is between about 0.5 microns and about 20 microns. Incertain embodiments, the mixture includes greater than 0% and less thanabout 80% by weight graphite particles. In further embodiments, thecomposite material includes about 40% to about 75% by weight graphiteparticles.

In certain embodiments, conductive particles which may also beelectrochemically active are added to the mixture. Such particlesprovide both a more electronically conductive composite as well as amore mechanically deformable composite capable of absorbing the largevolumetric change incurred during lithiation and de-lithiation. Incertain embodiments, the largest dimension of the conductive particlesis between about 10 nanometers and about 7 millimeters. All,substantially all, or at least some of the conductive particles maycomprise the largest dimension described herein. In further embodiments,the average or median largest dimension of the conductive particles isbetween about 10 nm and about 7 millimeters. In certain embodiments, themixture includes greater than zero and up to about 80% by weightconductive particles. In further embodiments, the composite materialincludes about 45% to about 80% by weight conductive particles. Theconductive particles can be conductive carbon including carbon blacks,carbon fibers, carbon nanofibers, carbon nanotubes, etc. Many carbonsthat are considered as conductive additives that are notelectrochemically active become active once pyrolyzed in a polymermatrix. Alternatively, the conductive particles can be metals or alloysincluding copper, nickel, or stainless steel.

In certain embodiments, an electrode can include a composite materialdescribed herein. For example, a composite material can form aself-supported monolithic electrode. The pyrolyzed carbon phase (e.g.,hard carbon phase) of the composite material can hold together andstructurally support the particles that were added to the mixture. Incertain embodiments, the self-supported monolithic electrode does notinclude a separate collector layer and/or other supportive structures.In some embodiments, the composite material and/or electrode does notinclude a polymer beyond trace amounts that remain after pyrolysis ofthe precursor. In further embodiments, the composite material and/orelectrode does not include a non-electrically conductive binder. Thecomposite material may also include porosity. For example, the porositycan be about 5% to about 40% by volume porosity.

In certain embodiments, an electrode in a battery or electrochemicalcell can include a composite material described herein. For example, thecomposite material can be used for the anode and/or cathode. In certainembodiments, the battery is a lithium ion battery. In furtherembodiments, the battery is a secondary battery, or in otherembodiments, the battery is a primary battery.

Furthermore, the full capacity of the composite material may not beutilized during use of battery to improve life of the battery (e.g.,number charge and discharge cycles before the battery fails or theperformance of the battery decreases below a usability level). Forexample, a composite material with about 70% by weight siliconparticles, about 20% by weight carbon from a precursor, and about 10% byweight graphite may have a maximum gravimetric capacity of about 2000mAh/g, while the composite material may only be used up to angravimetric capacity of about 550 to about 850 mAh/g. Although, themaximum gravimetric capacity of the composite material may not beutilized, using the composite material at a lower capacity can stillachieve a higher capacity than certain lithium ion batteries. In certainembodiments, the composite material is used or only used at angravimetric capacity below about 70% of the composite material's maximumgravimetric capacity. For example, the composite material is not used atan gravimetric capacity above about 70% of the composite material'smaximum gravimetric capacity. In further embodiments, the compositematerial is used or only used at an gravimetric capacity below about 50%of the composite material's maximum gravimetric capacity or below about30% of the composite material's maximum gravimetric capacity.

EXAMPLES

The below example processes for anode fabrication generally includemixing components together, casting those components onto a removablesubstrate, drying, curing, removing the substrate, then pyrolyzing theresulting samples. N-Methyl-2-pyrrolidone (NMP) was typically used as asolvent to modify the viscosity of any mixture and render it castableusing a doctor blade approach.

Example 1

In Example 1, a polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), graphite particles (SLP30 from Timcal corp.), conductive carbonparticles (Super P from Timcal corp.), and silicon particles (from AlfaAesar corp.) were mixed together for 5 minutes using a Spex 8000Dmachine in the weight ratio of 200:55:5:20. The mixture was then castonto aluminum foil and allowed to dry in a 90° C. oven, to drive awaysolvents, e.g., NMP. This is followed by a curing step at 200° C. in ahot press, under negligible pressure, for at least 12 hours. Thealuminum foil backing was then removed by etching in a 12.5% HClsolution. The remaining film was then rinsed in DI water, dried and thenpyrolyzed around an hour at 1175° C. under argon flow. The processresulted in a composition of 15.8% of PI 2611 derived carbon, 57.9% ofgraphite particles, 5.3% of carbon resulting from Super P, and 21.1% ofsilicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 2.

Example 2

In Example 2, silicon particles (from EVNANO Advanced Chemical MaterialsCo., Ltd.) were initially mixed with NMP using a Turbula mixer for aduration of one hour at a 1:9 weight ratio. Polyimide liquid precursor(PI 2611 from HD Microsystems corp.), graphite particles (SLP30 fromTimcal corp.), and carbon nanofibers (CNF from Pyrograf corp.) were thenadded to the Si:NMP mixture in the weight ratio of 200:55:5:200 andvortexed for around 2 minutes. The mixture was then cast onto aluminumfoil that was covered by a 21 μm thick copper mesh. The samples werethen allowed to dry in a 90° C. oven to drive away solvents, e.g., NMP.This was followed by a curing step at 200° C. in a hot press, undernegligible pressure, for at least 12 hours. The aluminum foil backingwas then removed by etching in a 12.5% HCl solution. The remaining filmwas then rinsed in DI water, dried and then pyrolyzed for around an hourat 1000° C. under argon. The process resulted in a composition of 15.8%of PI 2611 derived carbon, 57.9% of graphite particles, 5.3% of CNF, and21.1% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 3.

Example 3

In Example 3, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), and 325 mesh silicon particles (from Alfa Aesar corp.) weremixed together using a Turbula mixer for a duration of 1 hour in theweight ratios of 40:1. The mixture was then cast onto aluminum foil andallowed to dry in a 90° C. oven to drive away solvents, e.g., NMP. Thiswas followed by a curing step at 200° C. in a hot press, undernegligible pressure, for at least 12 hours. The aluminum foil backingwas then removed by etching in a 12.5% HCl solution. The remaining filmwas then rinsed in DI water, dried and then pyrolyzed around an hour at1175° C. under argon flow. The process resulted in a composition of 75%of PI 2611 derived carbon and 25% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC Oxide cathode. A typical cycling graph is shown inFIG. 4.

Example 4

In Example 4, silicon microparticles (from Alfa Aesar corp.), polyimideliquid precursor (PI 2611 from HD Microsystems corp.), graphiteparticles (SLP30 from Timcal corp.), milled carbon fibers (from FibreGlast Developments corp.), carbon nanofibers (CNF from Pyrograf corp.),carbon nanotubes (from CNANO Technology Limited), conductive carbonparticles (Super P from Timcal corp.), conductive graphite particles(KS6 from Timca corp.) were mixed in the weight ratio of20:200:30:8:4:2:1:15 using a vortexer for 5 minutes. The mixture wasthen cast onto aluminum foil. The samples were then allowed to dry in a90° C. oven to drive away solvents, e.g., NMP. This was followed by acuring step at 200° C. in a hot press, under negligible pressure, for atleast 12 hours. The aluminum foil backing was then removed by etching ina 12.5% HCl solution. The remaining film was then rinsed in DI water,dried and then pyrolyzed for around an hour at 1175° C. under argon. Theprocess resulted in a composition similar to the original mixture butwith a PI 2611 derived carbon portion that was 7.5% the original weightof the polyimide precursor.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 5.

Example 5

In Example 5, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), and silicon microparticles (from Alfa Aesar corp.) were mixedtogether using a Turbula mixer for a duration of 1 hours in the weightratio of 4:1. The mixture was then cast onto aluminum foil covered witha carbon veil (from Fibre Glast Developments Corporation) and allowed todry in a 90° C. oven to drive away solvents, e.g., NMP. This wasfollowed by a curing step at 200° C. in a hot press, under negligiblepressure, for at least 12 hours. The aluminum foil backing was thenremoved by etching in a 12.5% HCl solution. The remaining film was thenrinsed in DI water, dried and then pyrolyzed around an hour at 1175° C.under argon flow. The process resulted in a composition of approximately23% of PI 2611 derived carbon, 76% of silicon by weight, and the weightof the veil being negligible.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium nickel manganese cobalt oxide (NMC) cathode. A typicalcycling graph is shown in FIG. 6.

Example 6

In Example 6, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), graphite particles (SLP30 from Timcal corp.), and siliconmicroparticles (from Alfa Aesar corp.) were mixed together for 5 minutesusing a Spex 8000D machine in the weight ratio of 200:10:70. The mixturewas then cast onto aluminum foil and allowed to dry in a 90° C. oven, todrive away solvents (e.g., NMP). The dried mixture was cured at 200° C.in a hot press, under negligible pressure, for at least 12 hours. Thealuminum foil backing was then removed by etching in a 12.5% HClsolution. The remaining film was then rinsed in DI water, dried and thenpyrolyzed at 1175° C. for about one hour under argon flow. The processresulted in a composition of 15.8% of PI 2611 derived carbon, 10.5% ofgraphite particles, 73.7% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. The anodes where charged to 600mAh/g each cycle and the discharge capacity per cycle was recorded. Atypical cycling graph is shown in FIG. 7.

Example 7

In Example 7, PVDF and silicon particles (from EVNANO Advanced ChemicalMaterials Co), conductive carbon particles (Super P from Timcal corp.),conductive graphite particles (KS6 from Timcal corp.), graphiteparticles (SLP30 from Timcal corp.) and NMP were mixed in the weightratio of 5:20:1:4:70:95. The mixture was then cast on a copper substrateand then placed in a 90° C. oven to drive away solvents, e.g., NMP. Theresulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC Oxide cathode. A typical cycling graph is shown inFIG. 8.

Example 8

Multiple experiments were conducted in order to find the effects ofvarying the percentage of polyimide derive carbon (e.g. 2611c) whiledecreasing the percentage of graphite particles (SLP30 from Timcalcorp.) and keeping the percentage of silicon microparticles (from AlfaAesar corp.) at 20 wt. %.

As shown in FIGS. 9A and 9B, the results show that more graphite andless 2611c was beneficial to cell performance by increasing the specificcapacity while decreasing the irreversible capacity. Minimizing 2611cadversely affected the strength of the resultant anode so a value close20 wt. % can be preferable as a compromise in one embodiment.

Example 9

Similarly to example 8, if 2611c is kept at 20 wt. % and Si percentageis increased at the expense of graphite particles, the first cycledischarge capacity of the resulting electrode is increased. FIG. 10shows that a higher silicon content can make a better performing anode.

Example 10

When 1 mil thick films of polyimide are pyrolized and tested inaccordance with the procedure in Example 1. The reversible capacity andirreversible capacity were plotted as a function of the pyrolysistemperature. FIG. 11 indicates that, in one embodiment, it is preferableto pyrolyze polyimide films (Upilex by UBE corp) at around 1175° C.

Additional Examples

FIG. 12 is a photograph of a 4.3 cm×4.3 cm composite anode film withouta metal foil support layer. The composite anode film has a thickness ofabout 30 microns and has a composition of about 15.8% of PI 2611 derivedcarbon, about 10.5% of graphite particles, and about 73.7% of silicon byweight.

FIGS. 13-18 are scanning electron microscope (SEM) micrographs of acomposite anode film. The compositions of the composite anode film wereabout 15.8% of PI 2611 derived carbon, about 10.5% of graphiteparticles, and about 73.7% of silicon by weight. FIGS. 13 and 14 showbefore being cycled (the out-of-focus portion is a bottom portion of theanode and the portion that is in focus is a cleaved edge of thecomposite film). FIGS. 15, 16, and 17 are SEM micrographs of a compositeanode film after being cycled 10 cycles, 10 cycles, and 300 cycles,respectively. The SEM micrographs show that there is not any significantpulverization of the silicon and that the anodes do not have anexcessive layer of solid electrolyte interface/interphase (SEI) built ontop of them after cycling. FIG. 18 are SEM micrographs of cross-sectionsof composite anode films.

II. Electrodes and Electrochemical Cells

As described above, anode electrodes currently used in the rechargeablelithium-ion cells typically have a specific capacity of approximately200 milliamp hours per gram (including the metal foil current collector,conductive additives, and binder material). Graphite, the activematerial used in most lithium ion battery anodes, has a theoreticalenergy density of 372 milliamp hours per gram (mAh/g). In comparison,silicon has a high theoretical capacity of 4200 mAh/g. Silicon, however,swells in excess of 300% upon lithium insertion. Because of thisexpansion, anodes including silicon typically expand as well and it isoften difficult to maintain sufficient adhesion between the silicon andthe current collector. In addition to this, the silicon-based anode maywrinkle due to the expansion of the anode and/or the friction betweenthe anode and the other parts of the cell. This wrinkling causes thebattery to swell in thickness and should be avoided in order for thebattery to have a high volumetric energy density. The wrinkling alsocauses the interface between layers (e.g. anodes, cathodes, andseparator) to be uneven causing uneven usage of the active materialwithin a cell and other issues due to nonuniform distances between theopposing active materials.

Described herein are new electrodes (e.g., anodes and cathodes),electrochemical cells, and methods of creating electrodes andelectrochemical cells that may include a carbonized polymer. Forexample, mixtures, carbon composite materials, and carbon-siliconcomposite materials described above and in U.S. Patent ApplicationPublication No. 2011/0177393, the entirety of which is herebyincorporated by reference, can be used in certain electrodes, cells, andmethods described below.

The electrode described herein is different from the electrodes used incertain conventional cells including by not limited to the followingways: (1) The active material portion is a solid film instead of being acoating that is coated in a liquid form onto the foil. (2) Theattachment substance is a substance that is not originally includedwithin the active material solid film before attachment. Certainconventional electrode coatings are attached to the current collectorfoil by a binder such as PVDF which is part of the electrode coatingitself. In some cases, another coating such as carbon is used tostabilize the interface between the active material coating and thecurrent collector. For example, carbon may be a component of theelectrode coating. Also, the material that adheres the coating to thecurrent collector is still the PVDF even in the case where there is acarbon coating on the current collector.

After the material (e.g., silicon composite material) has been formedinto a shape such as a film, the material can be used in anelectrochemical cell (e.g., battery). During use of the electrochemicalcell, the cell is cycled wherein the silicon composite material absorbsand desorbs lithium during charging and discharging of the cell.Therefore, a silicon composite material can result in relatively largevolumetric changes during absorption and desorption of lithium sincesilicon can swell in excess of 300% upon lithium insertion, as describedabove. The silicon composite material can be formed into films (e.g.,sheets), and the absorption of lithium and resulting increase in volumeof the film can result in wrinkling of the film. FIG. 19 is a photographof an example of a film with wrinkles. In certain embodiments, the filmhas a thickness of about 10 to about 150 microns, and in furtherembodiments, the film has a thickness of about 15 to about 45 microns.Wrinkles in the film can result in non-uniform lithiation of theelectrochemically active material (e.g., silicon composite material).The film may also be coupled or attached to a current collector (e.g.,copper sheet). Wrinkling of the film can, for example, result indelamination of the film from the current collector and loss of abilityto collect electrical current.

Described below are methods of forming the film that results in nowrinkling or substantially no wrinkling of the film during cycling orlithiation. In addition, methods of attaching the film to a currentcollector are described as well as methods of attaching an electrode(e.g., anode and cathode) to a separator. Each of these methods can beused individually or in combination with the other methods to improveperformance of an electrochemical cell.

Electrodes Including an Electrode Attachment Substance to Adhere a Filmof Electrochemically Active Material to a Current Collector

An attachment (e.g., adhesive) substance can be used to couple or adherea film that includes electrochemically active material (e.g., siliconcomposite material) to a current collector (e.g., copper sheet or foil).The electrode attachment substance can adhere the film and currentcollector together to prevent delamination between them. The electrodeattachment substance can be placed or sandwiched between the film andthe current collector to form the electrode. Therefore, the electrodecan include the film, the attachment substance, and the currentcollector. In addition, the electrode can include a film with anelectrochemically active material on both sides of the currentcollector. For example, a first electrode attachment substance can besandwiched between a first film with an electrochemically activematerial and a first side of the current collector, and a secondelectrode attachment substance can be sandwiched between a second filmwith an electrochemically active material and a second side of thecurrent collector.

The film may include porosity such as discussed in section I.Embodiments may include porosity of about 1% to about 70% and/or about5% to about 50% by volume porosity. The electrode attachment substancemay at least partially be absorbed into the porosity such that at leastsome of the electrode attachment substance is within the porosity of thefilm. Without being bound by theory, the electrode attachment substancemay be absorbed into the porosity by capillary action. For example, asolution with the electrode attachment substance can be absorbed intothe porosity, and the solution can be dried, leaving the attachmentsubstance within the porosity of the film. The electrode attachmentsubstance within the porosity of the film can increase the mechanicaldurability of the film. As such, the film can result in a composite filmthat includes the electrode attachment substances. Furthermore, theelectrode attachment substance may extend through the entire thicknessof the film. For example, a substantial portion of the porosity may beopen such that the film is permeable to a solution that includes theelectrode attachment substance. Thus, the electrode attachment substancemay be a continuous phase within the film. In other embodiments, theelectrode attachment substance may only extend partially through or intothe thickness of the film.

In certain embodiments, the electrode attachment substance issubstantially electrically nonconductive (e.g., the electrode attachmentsubstance has an electrically conductivity such that, in use of theadhesive substance in an electrochemical cell, the attachment substancedoes not conduct electricity). Although the electrode attachmentsubstance may be substantially electrically nonconductive, theelectrochemical cell can result in better performance than if theelectrode attachment substance was electrically conductive. Withoutbeing bound by theory, absorption of the electrode attachment substancemay result in portions of the film physically contacting the currentcollector.

The electrode attachment substance may be a polymer. In certainembodiments, the electrode attachment substance includes polyamideimide(PAI) or is PAI. In further embodiments, electrode attachment substanceincludes polyvinylidene fluoride (PVDF) or is PVDF, includescarboxymethyl cellulose (CMC) or is CMC, or includes polyacrylic acid(PAA) or is PAA. The electrode attachment substance may also be othermaterials that provide sufficient adhesion (e.g., bonding strength) toboth the current collector and the film that includes electrochemicallyactive material. Additional examples of chemicals that can be or beincluded in the electrode attachment substance include styrene butadienerubber (SBR), polypyrrole (PPy), poly(vinylidenefluoride)-tetrafluoroethylene-propylene (PVDF-TFE-P), polyacrylonitrile,polytetrafluoroethylene, polyhexafluoropropylene, polyethylene oxide,polypropylene oxide, polyphosphazene, polysiloxane, polyvinyl fluoride,polyvinyl acetate, polyvinyl alcohol, polymethylmethacrylate,polymethacrylic acid, nitrile-butadiene rubber, polystyrene,polycarbonate, and a copolymer of vinylidene fluoride and hexafluoropropylene. The electrode attachment substance may be a thermoset polymeror a thermoplastic polymer, and the polymer may be amorphous,semi-crystalline, or crystalline.

Pressure may be applied to press the current collector and the filmtogether with the electrode attachment substance between. In certainembodiments, significant reduction in wrinkling is achieved whenpressures above 1 bar are applied and better results may occur whenpressure above 2 bars is applied. Pressure can be applied, for example,by putting the film, electrode attachment substance, and currentcollector through rolls such as calendaring rolls.

Another advantage to using an electrode attachment substance between thefilm and the current collector is that the complete assembly can be moreflexible than the film without the current collector and attachmentsubstance. For example, in certain embodiments, the film can be brittleand cannot be deformed (e.g., bent) significantly without cracking andfailure of the film. When the same film is coupled with or attached to acurrent collector with the electrode attachment substance, the completeassembly can be bent or deformed to a further extent compared to a filmthat is not coupled with or attached to a current collector withoutcracking or failure of the film. In certain embodiments, an electrodewhere the minimum radius of curvature before cracking is about 7 mm canbe wrapped around a radius of about 1 mm after the attachment andcalendaring has taken place. Advantageously, the complete electrodeassembly can be rolled to form a rolled-type (e.g., wound) battery.

There are a number of methods of making an electrode with an electrodeattachment substance adhering the film and current collector together.Described below are a number of examples. In certain examples, asolution of an electrode attachment substance and a solvent is made. Forexample, the electrode attachment substance can include PAI and thesolvent can include N-methyl pyrrolidone (NMP). The solution can beapplied or coated onto the current collector and/or the film. In certainembodiments, the coating of solution has a thickness of about 1 μm toabout 100 For example, the coating of solution may have a thickness ofabout 50 The film and current collector can then be placed into contactwith one another such that the solution is sandwiched between the filmand current collector. As described above, at least some of the solutionmay be absorbed into porosity of the film. Since the solution may beabsorbed into the film, the amount of solution coated onto the currentcollector or film may vary depending on the thickness and porosity ofthe film. Excess solution may be blotted using an absorbent material.The solution can then go through one or more drying steps to remove thesolvent from the solution leaving the electrode attachment substance.

Polymer Cell Construction

A cell attachment substance can also be used to couple or adhere theelectrode (e.g., anode and cathode) to a separator. The cell attachmentsubstance can adhere the electrode and the separator together to preventdelamination between them. The cell attachment substance can be placedor sandwiched between the electrode and the separator. The cellattachment substance may include any electrode attachment substancesdescribed above. For example, the cell attachment substance may includePVDF or be PVDF, may include PAI or be PAI, or may include or be CMC.

Since ions pass through the separator between the anode and cathode, thecell attachment substance also allows ions to pass between the anode andcathode. Therefore, the cell attachment substance can be conductive toions or porous so that ions can pass through the cell attachmentsubstance.

Furthermore, a solution may be made with the cell attachment substanceand a solvent and methods of attaching the electrode to the separatormay be similar to that described for attaching the film and currentcollector with the electrode attachment substance. Described below alsoare a number of examples.

Methods of Using Pressure to Form Silicon Composite Materials andMethods to Ensure that Cells that Include Silicon Composite Materialsare Kept Free of Wrinkles, Flat, and Thin

In certain embodiments, pressure is applied to the cell during formationof the carbon composite material. A mixture can be cast onto a substrateto form a coating on the substrate. The mixture can then be dried toremove solvents, and the dried mixture (e.g., precursor film) can thenbe cured. The precursor film then goes through pyrolysis to convert theprecursor film to the final composite film (e.g., anode film). In someembodiments, the precursor film is heated to about 900 to 1350° C. Whilethe precursor film is being pyrolysed, pressure can be applied to thefilm. In certain embodiments, a pressure greater than about 2 bars isapplied.

Advantageously, films that are pyrolysed under pressure remain flat andsubstantially wrinkle free during cycling or lithiation. Even withoutthe pressure applied during lithiation, the films can remainsubstantially free of wrinkles. Although, in certain embodiments,pressure may be applied during the initial formation (e.g., firstcharge) and initial cycling of the electrochemical cell as well asduring pyrolysis of the mixture. Without being bound by theory, it isbelieved that the wrinkles form during the first expansion and first fewcycles of the electrode and that there is delamination/wrinkling thatoccurs during that first charge and first few cycles. Applying pressureto prevent delamination may be a reason the pressure prevents wrinklingand swelling.

EXAMPLES

The following examples are provided to demonstrate the benefits of theembodiments of electrodes and electrochemical cells. These examples arediscussed for illustrative purposes and should not be construed to limitthe scope of the disclosed embodiments.

Electrode with an Electrode Attachment Substance

Attaching a film with an electrochemically active material to thecurrent collector can be performed by the following example methods.Polyamideimide (PAI) (e.g., Torlon 4000 series from Solvay) is dissolvedin a solvent (N-methylpyrrolidone (NMP), dimethylacetamide (DMAC),etc.). In one example, a solution with 10 w.t.% PAI can be prepared bymixing 10 g of PAI (e.g., Torlon 4000T-HV) with 90 g of NMP in a glassbottle. The solution is mixed until a transparent PAI solution isobtained. The solution may be mixed at room temperature for about 30minutes and then at 150° C. for about another 3 hours. The bottle may becovered by aluminum foil for better dissolving. Furthermore, the mixingcan be done, for example, using a magnetic stirring bar.

An example of assembling an electrode using the PAI solution includesproviding a 40×17 cm piece of copper foil with a thickness of about 10μm. Alcohol such as ethanol or IPA can be applied to a glass table, andthe copper foil can be placed over the alcohol onto the glass table.Pressure can be applied and a kimwipe can be wiped over the copper foilto remove any bubbles and excess alcohol between the copper foil and theglass table. A solution with the attachment substance such as a PAIsolution can then be applied to the copper foil. For example, 4 ml ofthe solution can be applied. A caster with a gap of about 50 μm can beused to form a uniform coating of the solution over the copper foil. Oneor more films with an active material can be placed onto the solution. Alint-free cloth can be used to remove excess adhesive. The copper foilcan then be removed from the glass table.

A second film with an active material can attached to the opposite sideof the copper foil by placing the copper foil with the first film downon a glass table. Then a similar procedure can be used as describedabove to apply the solution with the attachment substance and the secondfilm.

The assembly of the copper foil, solution, and film can be dried forabout two hours at about 110° C. and then dried in a vacuum oven forabout one hour at about 110° C. The drying removes the solvent leavingthe attachment substance between the film and the copper foil. The filmand copper foil can be pressed together during drying. For example, anadhesive loading of about 0.6 mg/cm² was used. Individual electrodes canthen be cut from the dried assembly.

Various attachment substances were used to create electrodes. FIG. 20 isa photograph of an anode without an electrode attachment substance in apressure cell after being cycled. The anode disintegrated almostcompletely. Table I lists a number of types of attachment substances(e.g., polymers) that were tested.

TABLE I Type of Content of polymer polymer (w.t. %) Results HSV-900 1.5,3.0 & 5.0 Depending on the HSV-900 content in solution but relativelyweak attachment Solef 6020 1.5, 3.0, 5.0 & 7.0 Depending on the Solef6020 content in solution but relatively weak attachment (better thanHSV-900) Solef 5130 1.5, 3.0, 5.0 & 7.0 Depending on the Solef 5130content in solution and showing the best attachment among PVDF solutionsSolef 5 (4 w.t. % Solef 5130 Depending on the ratio between Solef5130/PAI and 1 w.t. % PAI) & 5 5130 and PAI content in solution but (3w.t. % Solef 5130 relatively weak attachment, showing and 2 w.t. % PAI)good performance with Solef 5130/ PAI (3/2)

FIG. 21 is a photograph of an anode with Solef 5130 (e.g., PVDF) afterbeing cycled. Although this anode adhered to the current collectorbetter than without an attachment substance, a substantial portion ofthe film with the electrochemically active material disintegrated withcycling. When using solution with low PVDF content, there were no largedifference in attachment according to kind of PVDF and its content

In contrast to PVDF, the PAI attachment substance provided robustadhesion after exposure to electrolyte. FIG. 22 is a photograph of ananode with a PAI attachment substance after being exposed toelectrolyte.

To ensure that PAI did not adversely affect the electrochemicalperformance of the cells, cells were built with PAI, PVDF, and pressure,and subjected to rate characterization tests and long-term cycling atdifferent rates. FIG. 23 is a plot of discharge rate as a function ofcycles. The rate characterization test showed considerable variationwithin each group, but demonstrated that PAI-attached cells have similarrate capability compared to pressure cells, as shown in FIG. 23. Toomany of the PVDF cells in this group failed, so meaningful conclusionswere not made regarding the difference between the electrochemicalperformance of PAI and PVDF.

FIGS. 24 and 25 are plots of the discharge capacity as percentage of 8thdischarge capacity as a function of cycles, and FIG. 26 is a plot ofdischarge capacity as percentage of 2nd discharge capacity as a functionof cycles. The long term cycling tests showed PAI cells outperformingpressure cells, particularly at low rates, as shown in FIGS. 24 and 25.The tests also showed little difference between different types of PAI,and a large improvement over PVDF, as shown in FIG. 26. In summary,PAI-attached anodes are less disintegrated and have better cyclingperformance than anodes attached with any other method tested. Adifferent approach with PVDF could lead to better results withPVDF-attached anodes.

Additional methods of application of an electrode attachment substancewere tested. Table II lists methods of forming an electrode with anelectrode attachment substance along with the results.

TABLE II Method type Method details Result Conventional Casting on theglass plate with doctor blade Partial and weak using proper solution.attachment depending on Getting wet one side of Cu foil on the cast thePVDF content in solution. solution Putting anode on the wet side. Dryingat oven for 1 hour. Drying at vacuum oven for 1 hour. ConventionalPreparing Cu foil and anode attachment using Better attachment than &cold conventional method. conventional method pressing Cold calenderinganode-attached Cu foil sandwiched between two polypropylene (PP) plateusing roll press at room temperature. Conventional Preparing Cu foil andanode attachment using Better attachment than & hot conventional method.conventional method pressing Cold calendering anode-attached Cu foiland/or conventional & cold sandwiched between two PP plate using rollpressing press at temperature at 130° C. Dry type Dipping Cu foil intoproper PVDF solution. Better attachment than attachment Drying at roomtemperature. conventional method Hot calendering anode-attached Cu foiland/or conventional & cold sandwiched between two Kapton films usingpressing roll press at temperature at 130° C.

Using PAI (polyamideimide) to attach films with electrochemically activematerial (e.g., composite films) to copper foils (e.g., currentcollector) to form electrodes (e.g., anode or cathode) has been shown toprevent delamination of the electrode film from the copper foil. Theelectrode films typically will exhibit some volume expansion in the x-ydirection (e.g., the plane of the film rather than the thicknessdirection of the film) when lithiated (e.g., charged). The dimensionexpansion in plane of silicon composite-based electrode films may bereduced to essentially zero (e.g., about zero) to about 10% whenattachment using PAI is used. In addition to the reduction in x-yexpansion (e.g., in plane expansion), the electrodes (e.g., the assemblyof the film with electrochemically active material attached to thecopper foil) show essentially no failure or at least no significantfailure. For example, failure can include delamination between the filmand the copper foil and/or breakage of the current collector. Therefore,the electrode attachment substance may allow for expansion of the anodeactive material and current collector without significant breakage ofthe current collector and without significant delamination of the filmfrom the current collector. In cases where an inferior adhesion layer isused, the film can be peeled off the copper foil after disassembly of acell that has been charged. In the case of PAI, the electrode assemblyremains intact. Without being bound by theory, it is believed that thePAI attachment is superior to other approaches because of the affinity(e.g., better adhesion) of PAI to the film and the copper foil. PAI hasbeen used in lithium-ion batteries for other applications such as apolymeric binder of a silicon particulate electrode as described by N.Choi et al., “Enhanced electrochemical properties of a Si-based anodeusing an electrochemically active polyamide imide binder,” Journal ofPower Sources 177 (2008) 590-594. In addition to the physical attachmentenhancement, cells built with PAI-attached electrode assemblies haveshown better testing results compared to cells with silicon-containingself-supported electrode films adhered to the current collector using adifferent polymer such as PVDF.

Initial cell discharge rate capability tests show a higher ratecapability compared to cells made with different attachment methods.Table III includes data for the PAI-attached anode assembly cells.Direct control cells made using anode active material films attached tocopper with PVDF had a 2C discharge capacity of about 45%.

TABLE III C/5 C/2 C 2 C Type of Attachment Discharge Discharge DischargeDischarge Used Capacity Capacity Capacity Capacity 20% PAI in NMP 100%96.41% 90.36% 70.12% 15% PAI in NMP 100% 96.18% 89.39% 58.36% 20% PAI inDMAc 100% 96.19% 88.62% 61.86% 15% PAI in DMAc 100% 95.96% 88.69% 53.83%

In addition to discharge capacity, testing has shown an enhancement ofup to three times in cycle life in some cases for the cycle life ofcells built with electrode assemblies attached with PAI versus PVDF. ThePVDF-attached electrode assemblies were superior in performance than theother materials that had been tested other than PAI.

As can be seen in the photograph of FIG. 22, the anode active materialfilm is attached well to the current collector without any expansionafter cycling and disassembly. In addition, carboxymethyl cellulose(CMC) can be used as an electrode attachment substance that may yieldsimilar results. Other examples of possible attachment substancesinclude polyimide, epoxy, conductive glue, Na-CMC, PAI, etc.Furthermore, treating the copper foil surface (e.g. roughening, plasmatreatment) may further improve adhesion of the film to the copper foil.Describe above are various attachment substances and methods ofattaching the film to the copper foil. Each of the attachment substancesand methods can be used in various combinations.

Electrochemical Cell (Cell Attachment Substance to Adhere Together anElectrode and a Separator)

Attaching an electrode to a separator can be performed by the followingexample methods. A separator coating solution that includes a solventand an attachment substance can be prepared. A mixture can be made ofNMP (630.4 g) and EtOH (157.6 g) with a ratio of 80:20 (other possibleratios are 90:10 to 70:30). PVDF polymer (Solef 6020, 12 g) is added tothe mixture to form a solution. The solution can be mixed at roomtemperature for about 1 hour with magnetic stirrer and then heated toabout 150° C. and mixed until solution is transparent (about 30 mins). Aseparator can be cut to size and held in a fixture during thedipping/coating process. The separator can be any type of polyolefinseparator such as Celgard 2500 and EZ1592. The separator is dipped intothe PVDF solution bath and removed. Excess PVDF solution can be removed,and the separator can be dipped into a water bath for 5 minutes. Theseparator can then be dried for about 4 hours at room temperature andthen dried in a vacuum oven at 60° C. for about 6 hours (about 30 inHg).

The anode dipping solution that includes a solvent and an attachmentsubstance can be prepared. A mixture can be made of acetone (506.3 g),NMP (17.5 g) and EtOH (58.2 g) with a ratio of 87:3:10 (other possibleratios are 85-87:0-3:10). PVDF polymer (Solef 6020, 18 g) is added tothe mixture to form a solution. The solution can be mixed at roomtemperature for about 1 hour with magnetic stirrer and then heated toabout 220° C. and mixed until solution is transparent (about 30 mins).An anode can be held in a fixture during the dipping/coating process.The anode is dipped into the PVDF solution bath and removed. Excess PVDFsolution can be removed. The anode can then be dried in a vacuum oven at110° C. for about 1 hour.

The cell (e.g., pouch cell) can then be assembled using the separatorand anode. The cell can be hot pressed at about 110° C. for about 1 minfor a cell with a thickness of about 1.8 mm and about 2 min for a cellwith a thickness of about 4.5 mm. After hot pressing, the cell is movedimmediately to a cold press at room temperature for a similar time asused with the hot press. A spacer can be used to adjust the gap in thetop and bottom plates of the press to avoid crushing the cell. Thepressing consolidates the PVDF coating on the separator and the PVDFcoating on the anode.

Various cell performance comparisons were made between a polymer cell(e.g., PVDF coatings) and non-polymer cells (e.g., no attachmentsubstance). All the cells used similar anodes. The cells were kept inaluminum clamps and then unclamped. FIG. 27 is a plot of dischargecapacity as a function of cycles. The polymer cells showed nosignificant degradation while the non-polymer cells showed immediatedegradation. FIG. 28 is a plot of discharge capacity as a function ofcycles for two different separators. Separator 1 is Celgard 2500 andSeparator 2 is Celgard EZ1592. The cycle performance did not vary muchbetween the two separator types. FIG. 29 is a plot of discharge capacityas a function of cycles for two different electrolyte solutions.Electrolyte 1 is a EC:DEC:DMC (1:1:1) based electrolyte and Electrolyte2 is a EC:EMC (3:7) based electrolyte. The cycle performance did notvary much between the two electrolytes.

Additional solutions of polymers and solvents were tested. Table IVlists various solutions and results.

TABLE IV Type of polymer content and solution Solution compositionStability Method Results 6 w.t. % HSV- Low Two heavy Too thick coatinglayer 900 in Ac/EtOH dryers for 1 Broad distribution of coating layer(90/10) min 10 sec Wrinkles and uneven parts in bottom Partial peel-offproperties between separator and coating layer Bad performance duringfast characterization 6 w.t. % Kynar Low Two heavy Too thick coatinglayer 760 in dryers for 1 Broad distribution of coating layerAc/EtOH(90/10) min 10 sec Wrinkles and uneven parts in bottom Peel-offproperties between separator and coating layer Bad performance duringfast characterization 6 w.t. % K301-F Low Two heavy Too thick coatinglayer in dryers for 1 Broad distribution of coating layer Ac/EtOH(90/10)min 10 sec Wrinkles and uneven parts in bottom Bad performance duringfast characterization 4 w.t. % HSV- Low Two heavy Relatively thickcoating layer 900 in Ac/EtOH dryers for 1 Broad distribution of coatinglayer (90/10) min 10 sec Wrinkles and uneven parts in bottom Peel-offproperties between separator and coating layer Bad performance duringfast characterization 4 w.t. % K301-F Low Two heavy Relatively thickcoating layer in dryers for 1 Broad distribution of coating layerAc/EtOH(90/10) min 10 sec Wrinkles and uneven parts in bottom 3 w.t. %HSV- Low Two heavy Relatively thick coating layer 900 in Ac/EtOH dryersfor 1 Broad distribution of coating layer (90/10) min 10 sec Wrinklesand uneven parts in bottom Bad performance during fast characterization3 w.t. % K301-F Low Two heavy Relatively thick coating layer in dryersfor 1 Broad distribution of coating layer Ac/EtOH(90/10) min 10 secWrinkles and uneven parts in bottom Bad performance during fastcharacterization 2 w.t. % HSV- Low Two heavy Too thin coating layer 900in Ac/EtOH dryers for 1 Broad distribution of coating layer (90/10) min10 sec Wrinkles and uneven parts in bottom Bad performance during fastcharacterization 2 w.t. % K301-F Low Two heavy Too thin coating layer indryers for 1 Broad distribution of coating layer Ac/EtOH(90/10) min 10sec Wrinkles and uneven parts in bottom Bad performance during fastcharacterization 4 w.t. % Solef Low 1st generation Good coating layerbut still show broad 6020 in air blade for distribution of coating layerAc/EtOH(90/10) cold drying Wrinkles and uneven parts in bottom (10 sec)and Better than Heavy dryer only system heavy dryer Moderate performanceduring fast for hot drying characterization (1 min) 5 w.t. % Solef Low1st generation Good coating layer but still show broad 6020 in air bladefor distribution of coating layer Ac/EtOH(90/10) cold drying Wrinklesand uneven parts in bottom (10 sec) and Better than Heavy dryer onlysystem heavy dryer Moderate performance during fast for hot dryingcharacterization (1 min) 3 wt. % Solef Low 2nd generation Moderatecoating layer but easy to peel- 5130 in air blade for off propertyAc/EtOH cold drying Wrinkles and uneven parts in bottom (80/20) (10 sec)and Better than 1st generation air blade system heavy dryer Moderateperformance during fast for hot drying characterization (1 min) 3 wt. %Solef Moderate 2nd generation Moderate coating layer but easy to peel-6020 in air blade for off property Ac/NMP/EtOH cold drying Wrinkles anduneven parts in bottom (88/2/10) (10 sec) and Better than 1st generationair blade system heavy dryer Moderate performance during fast for hotdrying characterization (1 min) 3 wt. % Solef Good 2nd generation Goodcoating layer 6020 in air blade for Still having wrinkles and unevenparts in Ac/NMP/EtOH cold drying bottom (87/3/10) (10 sec) and Betterthan 1st generation air blade system heavy dryer Moderate performanceduring fast for hot drying characterization (1 min) 3 wt. % Solef Low2nd generation Good coating layer 6020/Solef 5130 air blade for Stillhaving wrinkles and uneven parts in (75/25) in cold drying bottomAc/EtOH (9/1) (10 sec) and Better than 1st generation air blade systemheavy dryer Moderate performance during fast for hot dryingcharacterization (1 min) 3 wt. % Solef Moderate 2nd generation Goodcoating layer 6020/Solef 5130 air blade for less wrinkles and unevenparts compared (75/25) in cold drying with Ac/EtOH and/or Ac/NMP/EtOH(10 sec) and Ac/NMP/EtOH(88/2/10) (88/2/10) heavy dryer Better than 1stgeneration air blade system for hot drying Moderate performance duringfast (1 min) characterization 2 wt. % Solef Good 2nd generation Goodcoating layer but too thin 6020/Solef 5130 air blade for less wrinklesand uneven parts compared (75/25) in cold drying with Ac/EtOH and/orAc/NMP/EtOH (10 sec) and Ac/NMP/EtOH(88/2/10) (88/3/10) heavy dryerBetter than 1st generation air blade system for hot drying Badperformance during fast (1 min) characterization 2 wt. % Solef Good 2ndgeneration Good coating layer but too thin 6020/Solef 5130 air blade forless wrinkles and uneven parts compared (50/50) in cold drying withAc/EtOH and/or Ac/NMP/EtOH (10 sec) and Ac/NMP/EtOH(88/2/10) (87/3/10)heavy dryer Better than 1st generation air blade system for hot dryingBad performance during fast (1 min) characterization 2 wt. % Solef Good2nd generation Good coating layer but too thin 6020/Solef 5130 air bladefor less wrinkles and uneven parts compared (25/75) in cold drying withAc/EtOH and/or Ac/NMP/EtOH (10 sec) and Ac/NMP/EtOH(88/2/10) (87/3/10)heavy dryer Better than 1st generation air blade system for hot dryingBad performance during fast (1 min) characterization

Various pressures and heat were tested after cell assembly andelectrolyte addition. This pressure and heat is what seals the coatingson the anode and separator together. Table V lists various pressingprocesses and the results.

TABLE V Temperature conditions during pressing Other conditions Results120 C. for hot 30 sec for single layer Moderate adhesive property presswithout cell between anode and separator cold press step 2 min for 15layer cell and/or cathode and separator But, bad cell performance 120 C.for hot 30 sec for single layer Moderate adhesive property pressing &cell between anode and separator cold press 2 min for 15 layer celland/or cathode and separator with weight With moderate cell performanceCold pressing 30 sec for single layer Bad adhesive property between cellanode and separator and/or cathode and separator And, bad cellperformance Hot pressing & 30 sec for single layer Good adhesiveproperty cold pressing cell between anode and separator with 1 min for 5layer cell and/or cathode and separator 2 min for 15 layer cell withoutkind of cells With moderate cell performance 1 min for single layer Goodadhesive property cell: severe time frame between anode and separator tocheck damage of and/or cathode and separator separator But, bad cellperformance 1 min for 5 layer cell Good adhesive property with thehighest between anode and separator pressure to check and/or cathode andseparator damage of separator But, bad cell performance

A polymer cell was also assembled by attaching electrodes to apolymer-coated separator that had not been fully dried (still hassolvents). Attachment between the polymer coated separator andanode-attached Cu foil or cathode was achieved by directly addinganode-attached Cu foil or cathode and adhering the electrode to thesurface of a solvated PVDF-coated separator. Good attachment wasobserved between the separator and anode and/or separator and cathodebut poor cell performance was measured. Without being bound by theory,the poor performance was likely due to excess PVDF that filled the poresin the separator and electrode.

Other types of coating methods can be used such as spray, electro-spray,electrospinning, casting with doctor blade, etc. Other types ofseparators can also be used such as metal oxide-coated or metal oxidefilled separator.

Pressure Methods to Prevent Anode Wrinkling in Silicon-Based Anodes

Upon lithiation, the silicon in the anode swells and expands. Since thematerial in the electrode is confined between a current collector andthe separator this swelling results in a wrinkly anode. FIG. 30 is aphotograph of an example wrinkly anode.

If the anode is left to expand freely, for example in a beaker cell, thematerial in the anode does not wrinkle, but instead it bulges outward.In this state, it is possible to measure the swelling of the anode layerin the vertical, which in anodes described herein increases from about33 microns (in the pristine state) to about 45 microns (after fullcharge).

The approach used to overcome the wrinkling problem can be twofold: theapplication of pressure to constrain the anode while preventing itslateral expansion, and the use of a blend of polymers to bond anode,cathode and separator together while allowing for movement of thelithium ions. Certain bonding materials are used in some commerciallyavailable polymer cells.

In previous attempts, in order to apply pressure onto the cell, the mainmechanism used was to sandwich the cell between two sheets ofpolypropylene, ⅛ of an inch thick, with green felt as interfacialmaterial between them. The stack was held together with paper clips fromthe sides. This pressure mechanism did not prevent the formation ofwrinkles.

In order to apply a higher pressure during the charging step, othersetups were tested. Several combinations of materials (polypropylene andaluminum) and interface materials (green felt and rubber) were used toeliminate anode wrinkling during cycling. Table VI summarizes theresults.

TABLE VI Interface Pressure Wrin- Plate material material method klesCell number Polypropylene Rubber Binder clips YES ML4209-4212 AluminumGreen felt C-clamps NO ML4205-4208 Aluminum Rubber C-clamps NOML4221-4224 Aluminum None C-clamps NO ML4237-4240

At high pressures, both green felt and rubber work well. It is importantto note that the force applied with the C-clamps is not very wellcontrolled, as it depends on the manual ability of the operator. In caseof the cells what were clamped without interfacial material (barealuminum), the cell stack was very tightly compressed, and theelectrodes did not even soak with electrolyte. This result is attributedto excess pressure being applied to the cell stack.

In terms of rate performance, the polymer cells constrained withc-clamps show a better performance than past polymer cells, assummarized in Table VII (values shown are the averages for the fourtested cells of each type).

TABLE VII Cap.@C/ Cap.@C/ Cap.@C Cap.@2 C Cell type 5(mAh) 2(mAh) (mAh)(mAh) PP/Rubber/binder 386.7257 276.17 138.684 88.65451 clips Al/greenfelt/c- 472.9861 459.085 443.9014 397.8245 clamps Al/rubber/c- 464.397458.62 444.6271 387.7365 clamps

In order to better understand the amount of pressure needed to preventwrinkling, different weights were applied to 5-layer cells sandwichedbetween aluminum plates (3×3 inch) with rubber pads (2×2 inch). Fourdata points were collected, with just one cell for each weight (25, 50,75 and 100 lb). Of those, only the cell with 100 pounds of force showedcomplete reduction of the wrinkling problem. FIGS. 31A-C are photographsof anodes for weights 100 lb, 75 lb, and 50 lb, respectively.

It is also worth mentioning that applying pressure during forming andthen moving the cell into a lower pressure device seems to be aneffective way to control wrinkling in the anode. This was done by usingthe Al/rubber/c-clamp setup during the first charge and discharge, andthen transfer the cell stack to a Polypropylene/green felt/binder clipssystem. FIG. 32 is a photograph of an anode which shows the absence ofwrinkles when this procedure is followed.

Various embodiments have been described above. Although the inventionhas been described with reference to these specific embodiments, thedescriptions are intended to be illustrative and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. An electrode comprising: a current collector; afilm in electrical communication with the current collector, the filmcomprising a carbon phase that holds the film together; and an electrodeattachment substance that adheres the film to the current collector. 2.The electrode of claim 1, wherein the film is a monolithicself-supporting film.
 3. The electrode of claim 1, wherein the filmfurther comprises silicon particles distributed within the carbon phase.4. The electrode of claim 1, wherein the carbon phase comprises hardcarbon.
 5. The electrode of claim 1, wherein the electrode attachmentsubstance is substantially electrically nonconductive.
 6. The electrodeof claim 1, wherein the electrode attachment substance comprises apolymer.
 7. The electrode of claim 1, wherein the electrode attachmentsubstance comprises polyamideimide.
 8. The electrode of claim 1, whereinthe electrode attachment substance comprises polyvinylidene fluoride. 9.The electrode of claim 1, wherein the electrode attachment substancecomprises polyacrylic acid.
 10. The electrode of claim 1, wherein thefilm comprises porosity and at least some of the electrode attachmentsubstance is within the porosity of the film.
 11. The electrode of claim10, wherein the porosity comprises about 5 to about 50 percent by volumeof the film.
 12. The electrode of claim 10, wherein the porositycomprises about 1 to about 70 percent by volume of the film.
 13. Theelectrode of claim 1, wherein the current collector is an electricallyconductive foil.
 14. The electrode of claim 1, wherein the electrodeattachment substance allows for expansion of the anode active materialand current collector without significant failure of the electrode. 15.The electrode of claim 1, wherein the electrode can be bent to a radiusof curvature of at least 7 mm without significant cracking.
 16. A methodof forming an electrode comprising: sandwiching an electrode attachmentsubstance between a current collector and a solid film comprisingelectrochemically active material such that the electrode attachmentsubstance adheres the solid film to the current collector and the solidfilm is in electrical communication with the current collector.
 17. Themethod of claim 16, wherein the solid film at least partially absorbsthe electrode attachment substance into porosity of the film.
 18. Themethod of claim 16, wherein the electrode attachment substance issubstantially electrically nonconductive.
 19. The method of claim 16,wherein the electrode attachment substance comprises a polymer.
 20. Themethod of claim 16, wherein the electrode attachment substance comprisespolyamideimide.
 21. The method of claim 16, wherein the electrodeattachment substance comprises polyvinylidene fluoride.
 22. The methodof claim 16, wherein the electrode attachment substance comprisespolyacrylic acid.
 23. An electrochemical cell comprising: a porousseparator sheet; and a cell attachment substance sandwiched between theporous separator sheet and the electrode of claim
 1. 24. Theelectrochemical cell of claim 23, wherein the cell attachment substancecomprises polyvinylidene fluoride.
 25. The electrochemical cell of claim23, wherein the cell attachment substance coats at least one of theporous separator sheet and the electrode.
 26. The electrochemical cellof claim 23, wherein the cell attachment substance coats the porousseparator sheet and the electrode.
 27. The electrochemical cell of claim26, wherein the cell attachment substance that coats the porousseparator sheet is a first cell attachment substance and the cellattachment substance that coats the electrode is a second cellattachment substance that is chemically different than the first cellattachment substance.
 28. A method of forming a electrochemical cellcomprising: sandwiching a cell attachment substance between a porousseparator sheet and the electrode of claim
 1. 29. The method of claim28, further comprising coating at least one of the porous separatorsheet and the electrode with the cell attachment substance.
 30. Themethod of claim 28, further comprising coating the porous separatorsheet and the electrode with the cell attachment substance.
 31. Themethod of claim 30, further comprising heating the cell attachmentsubstance after sandwiching the cell attachment substance between theporous separator sheet and the electrode.